Welcome back! Today, we're going to talk about Transits and the Kepler Satellite, and talk about how we use this new technique to study extrasolar planets. Well it's new in some ways, in many ways it's very old, really goes back to the study of the size of our own solar system. So, we're going to begin by talking about the Transit of Venus, then turn to Extrasolar Transits. Then, conclude by talking about some of the remarkable things that the Kepler Satellite has taught us about Extrasolar planets in the last few years. We begin with the Transit of Venus and take you back to the 1700s, at that point we knew Newton's Laws. We knew about Kepler and his work and we knew that the planets revolved around the sun. We knew the relative distance of the planets from the sun. We knew how to measure the distance from the sun to Mars in astronomical units, the distance from the Earth to the sun. We knew the distance to Jupiter in astronomical units, to Venus. We had all those pieces of the puzzle. What we didn't know was how far away the sun was. We didn't know how to convert from astronomical units to kilometers or meters. Back then they didn't have Google, so they couldn't look it up or more importantly, no one knew how to measure that. And it was a guy name Halley, you've probably heard of him from Halley's Comet, who in 1761, realized that it would be possible to use the transit of Venus. To measure as Venus moved across the sun, we can see how the transit of Venus appeared to different points on the earth and then used geometry to compute from the timing of the transit of Venus, exactly what the distance was to the sun. Now the problem was Halley at the time he realized this was already an old man and he also knew when the next transits would occur. They wouldn't occur for basically another 50 years. So what he did was he wrote a paper that described this method and looked ahead to the next generation of scientists who would be able to make that measurement. And these Venus transits come in pairs, basically 8 years apart separated by 120 years. So there was a pair of transits in 1761 and 1769. Another pair of transits in the 1880s, 1890s and there was a pair of transits that just happened in 2004 and 2012. And you can do the calculation and realize that you will all have to live a very long time to see the next transit of Venus. That if you have an idea on something we should do with the transit of Venus like Edmund Halley, you should write it down and leave it to our successors to carry that out. So what's the idea and why is it so rare? Well you'd think that Venus and the Earth would line up every orbit of Venus. The problem is Venus' orbit is tilted relative to the Earth's orbit by about 3.4 degrees and that's shown in this picture here. The Earth is moving around the Sun on this red ellipse. Venus is moving around the Sun, on this green ellipse and Venus passes through, the plane of the Earths orbit only twice. Once here that it dips below the plane of the Earth's orbit and then once here. In order for a transit to occur, when Venus is here Earth has to be at just the right position of its orbit. Most of the time when Venus passes through this point, Earth is here, or it's there, or it's there. It's not at the right position, so while Venus passes through the plane, twice every Venetian year, most of the time, the Earth isn't properly positioned. As I noted, only twice in every 120 years do the planets align, and the Earth is here, Venus is here, and we get to see Venus move across the sun. Why was that so important? Well, the reason why it was so important is you can then go to two different positions on the Earth. Here's observer A, here's observer B. We know how far apart they are on the Earth. Each of these observers watches Venus move in front of the Sun. For observer A over here, he'll observe Venus enter here, pass through here. Observer B here, she will observe Venus crossing the sun here and passing through here. The duration of the transit will depend on where you are on the planet. So we can time the duration with a very good clock, see when Venus enters, when Venus exits. And if we can measure them on many different positions, we could learn by doing relatively simple geometry what the distance is from the sun to the Earth. And this is what 18th century astronomers wanted to do. They wanted to measure the size of the solar system. In fact lots of people went on this and one of the famous people was Captain Cook. Let me actually go back to the 1761 attempt. In 1761, many different groups tried see this transit. There was a bunch of astronomers named Mason and Dixon, they went down to Cape Town South Africa, to observe the transit. There was a French group that went to Toblolsk. There was a group observing it on Rodriquez Island. Lomonosv tried it from St. Petersburg. Le Gentile traveled to Pondicherry, India. Unfortunately the 1761 transit took place during the Seven Years War, and the vagaries of war disrupted most of these measurements. This stopped most of these groups, some of these astronomers got arrested as they traveled, as Frenchmen in Russian territory or as Brits moving through waters controlled by the enemy nations. These events meant that astronomers in 1761 didn't get good enough data. This problem led to the first international scientific treaty. The world scientist realized that they're about to miss out on an important opportunity in 1769. That if they failed to observe the transit in 1769, they'd have to wait another nearly 120 years to see the next transit. This led to an agreement that would allow French astronomers to travel through British territory, British astronomers to travel through French territory, as long as they were carrying out an eclipse mission. In fact, one of the famous trips was Captain Cook's expedition. And this is from Captain Cook's log where you can see Captain Cook recording hand-drawn pictures of Venus's transit, entering the Sun and exiting the Sun. And Captain Cook's measurements, together with measurements of a lot of other astronomers. The guy Mason and Dixon, who did the measurements in 61 repeated this in 69. The British were so pleased with their work, they felt they made such a good team, they traveled to measure and map out the border between Maryland and Pennsylvania. And many Americans know of the Mason-Dixon Line which played an important role in the Civil War as marking the boundary between slave states and free states and it's the same Mason-Dixon. And, when the data came back in 1769 these 18th century astronomers did remarkably well. Here's Captain Cook's expedition, travelling all over, was one of the first Europeans to see Hawaii and explored much the Pacific during this time, and they got great data on the size of the Solar System. Lalande, taking the 1769 data, estimated at a distance of 153 million kilometers for the astronomical unit. That's remarkably good so based on this 18th century data, he measured the distance to a couple percent, he got a very accurate measurement. And Newcomb, using the next set of eclipses, In the 19th century, measured the distance to be 149.59 million kilometers. Incredibly close to the modern value and today we can achieve a very accurate measurement of the distance from the Earth to the Sun. And now our measurements are so sensitive we could notice the evolution that happens in the planetary orbits. But this value here is really impressive, these early astronomers did a remarkably good job in making these measurements. And these observations became the ruler that we have used to measure the size of our universe. So all of our measurements in astronomy basically start with this astronomical unit. We use that as our unit to measure the parsec. Our unit that we use to get the distance to the nearest stars and then we extrapolate on outwards. Going from a parsec to the megaparsec, the million parsec distance, to the nearest galaxies, out to the billions of parsec distances. The roughly tens of billions of parsecs distance we measure, to the edge of the visible universe and it really all starts with this 1769 eclipse expedition. So I'm going to answer a quick question or two on this, and then jump forward to modern times and see how we use transits today to discover the properties of extrasolar planets. Here's a picture of the transit of Venus taking place in 2004. Here's where the 2012 transit was and here's a question or two I'd like you to answer before returning back.